The NSA, the original primary developer of SELinux, released the first version to the open source development community under the GNU GPL on December 22, 2000.[6] The software was merged into the mainline Linux kernel 2.6.0-test3, released on 8 August 2003. Other significant contributors include Red Hat, Network Associates, Secure Computing Corporation, Tresys Technology, and Trusted Computer Solutions.
I’m not sure why that’s a problem. The NSA needed strong security so they created a project to serve the need. They are no longer in charge of SELinux but I wouldn’t be surprised if they still worked on it occasionally.
There are a lot of reasons to not like the NSA but SELinux is not one of them.
So, how many backdoors do you think they implemented into the kernel?
None
There are always exploits to be used. Also there isn’t a lot of use in kernel specific exploits